1. Field of the Invention
The present invention relates to a photovoltaic element and a method of producing the same. More particularly, the invention concerns a photovoltaic element that can be produced at a low manufacturing cost and that demonstrates a high photoelectric conversion efficiency and little photodeterioration, and a method of producing the same.
2. Related Background Art
Significant technological issues concerning the practical application of photovoltaic elements to the generation of electric power include reducing manufacturing cost and increasing photovoltaic element area. Low cost materials having high conversion efficiency have been investigated in this regard.
As such materials for the photovoltaic elements, there can be included, for example, Group IVA amorphous semiconductors of the tetrahedral type such as amorphous silicon (hereinafter referred to as amorphous Si or a-Si), amorphous silicon germanium (hereinafter referred to as amorphous SiGe or a-SiGe), amorphous silicon carbide (hereinafter referred to as amorphous SiC or a-SIC) , etc., Group II-VI compound semiconductors such as CdS, Cu.sub.2 S, etc., Group I-III-VI.sub.2 compound semiconductors such as CuInSe.sub.2, CuInGaSe, etc., and so on.
Among others, thin-film photovoltaic elements using an amorphous semiconductor for a photovoltaic energy generation layer have advantages in that a film thereof can be formed in a larger area, in a smaller thickness, and on an arbitrary substrate material, as compared with the single-crystal photovoltaic elements, and are thus promising for practical application.
However, increased photoelectric conversion efficiency and improved photodeterioration must be pursued in the practical application of the aforementioned photovoltaic elements using the amorphous semiconductor as elements to generate electric power.
With respect to increased photoelectric conversion efficiency, there are the following reports.
(a) A single cell (of a type having only one pin junction) was obtained which exhibited a photoelectric conversion efficiency over 13% (Miyachi et al., Extended Abstracts (The 53rd Autumn Meeting, 1992); The Japan Society of Applied Physics, 17p-B-5, p. 746). PA1 (b) One way to increase the photoelectric conversion efficiency is to improve the so-called doped layers such as the p-type semiconductor layer, the n-type semiconductor layer, etc. The doped layers are required to have small activation energy and little absorption of light. U.S. Pat. No. 4,109,271 discloses the technology in which amorphous silicon carbide (a-SiC) is used to expand the optical band gap of the doped layers, thus reducing the absorption of light. PA1 (c) It has been reported that microcrystal SiC is used for the doped layer to decrease the activation energy and the absorption of light (Y. Hattori et al., 3rd International Photovoltaic Science and Engineering Conference, 1987, p. 171). PA1 (d) U.S. Pat. No. 4,816,082 discloses the technology in which a gradient in the layer thickness direction is given for the band gap of the i-type semiconductor layer. PA1 (e) A method of reducing the localized states of the amorphous semiconductor layer. PA1 (f) A method of using an alloy of a Group IVA element for the amorphous semiconductor layer and adjusting its compositional ratio to a suitable value. PA1 (g) A method of forming a stacked photovoltaic element, thereby decreasing the absorption of light per unit cell and decreasing the thickness of the i-type semiconductor layers.
On the other hand, as methods for improving the photodeterioration, the following can be included.
As a consequence, there has recently been reported (S. Guha, 25th IEEE Photovoltaic Specialists Conference, 1996, p. 1017) that a triple stacked photovoltaic element was obtained which had a pin junction of a-Si film, a pin junction of a-SiGe film, and a pin junction of a-SiGe film stacked in order from the light incidence side and demonstrated a 10.4% photodeterioration rate 11.83% stabilization efficiency. However, there are desires for further reduction in the photodeterioration rate and further improvement in the stabilization efficiency.
In contrast, there has recently been reported a photovoltaic element which uses microcrystal silicon for the i-type semiconductor layer and exhibits little photodeterioration (J. Meier et al., IEEE First World Conference on Photovoltaic Energy Conversion, 1994, p. 409). This photovoltaic element was fabricated using plasma CVD by glow discharge, similar to the fabrication process of the conventional amorphous type photovoltaic elements. There is thus a possibility that a large-area photovoltaic element can be fabricated by an inexpensive fabrication process, as with the conventional amorphous type photovoltaic elements. However, the photoelectric conversion efficiency of this photovoltaic element is only 7.7% even in a recent report (D. Fischer et al., 25th IEEE Photovoltaic Specialists Conference, 1996, p. 1053), which is still inferior to those of the conventional amorphous silicon single cells (.about.13%). Therefore, the photovoltaic elements using microcrystal silicon for the i-type semiconductor layer have been an important technological subject in the pursuit of improvement in photoelectric conversion efficiency.
In addition, the photovoltaic elements using the microcrystal silicon for the i-type semiconductor layer have essential problems in that the band gap of microcrystal silicon is narrow and the open circuit voltage (Voc) of the photovoltaic element is as low as about 0.4-0.5 V.
As a method of solving these problems, there has been studied a method of stacking a photovoltaic element using microcrystal silicon for the i-type semiconductor layer and a photovoltaic element using amorphous silicon for the i-type semiconductor layer to form a stacked photovoltaic element, instead of using as a single cell the photovoltaic element using microcrystal silicon for the i-type semiconductor layer. In this way, the open circuit voltage (Voc) of the photovoltaic element is increased, and the photoelectric conversion efficiency is improved. As a result, there has recently been achieved the photoelectric conversion efficiency of 13.1% in the above-mentioned stacked photovoltaic element (D. Fischer et al., 25th IEEE Photovoltaic Specialists Conference, 1996, p. 1053). However, it was also reported in the reference that stacking of the photovoltaic element using amorphous silicon for the i-type semiconductor layer naturally resulted in photodeterioration and that after photodeterioration for 145 hours, the photoelectric conversion efficiency was 10% and the photodeterioration rate was 12%.
This value, a photodeterioration rate of 12%, cannot be said to be small, when compared with that of the photovoltaic element using only the amorphous semiconductor (for example, the one demonstrating the photodeterioration rate of 10.4% and the stabilization efficiency of 11.83% as described previously). Further, the element described in Fischer is also inferior in stabilization efficiency. It is believed that this result is due to the fact that in order to match the electric current generated in the microcrystal silicon with the electric current generated in the amorphous silicon, the film thickness of amorphous silicon with a large band gap but with a small absorption coefficient must be large, approximately 210 nm, whereby the photodeterioration of the amorphous silicon portion increases.
Heretofore, as a method for the production of microcrystal silicon, there has been employed, for example, a high frequency plasma CVD process using a frequency in the range of 13.56 MHz to 110 MHz (J. Meier et al., IEEE First World Conference on Photovoltaic Energy Conversion, 1994, p. 409), but this production method showed a very small deposition rate of microcrystals, approximately 0.1 nm/sec and was difficult to be practically applied to the fabrication of photovoltaic elements.
Further, as the production method of doped microcrystal SiC, there has been reported, for example, the ECR plasma CVD process employing the frequency of 2.45 GHz with application of a magnetic field (Y. Hattori et al., 3rd International Photovoltaic Science and Engineering Conference, 1987, p. 171), but this method had a problem that the underlying semiconductor layer was damaged. Therefore, i-type microcrystal SiC has never been made by this method.